1. Field of the Invention
The present invention relates to a thermal detector and a method of producing the same using a semiconductor heat sensor element such as a thermistor, which senses a fire by electrically detecting temperature.
2. Description of the Related Art
Those arrangements, for example disclosed in Japanese Patent Laid-Open Publication No. 1-259494 have been known as this type of thermal detector and its production method.
This type of thermal detector is first provided with a printed circuit board having a heat sensor element and a body for accommodating the printed circuit board. Also, a cover is provided so that the heat sensor element of the printed circuit board mounted on the body is caused to face the outside thereof. It is further constructed to have an inner cover placed between the printed circuit board and the cover, which is screwed to the printed board. The inner cover is provided for the purpose of positioning the heat sensor element which penetrates therethrough and of preventing dust from entering.
For example, FIG. 11 shows a conventially known type of this detector. Referring to FIG. 11, 1 is a heat sensor element using a thermistor or the like of which the resistance value varies according to temperature. This heat sensor element 1 has lead wire connected to a printed circuit board 4, which is incorporated into a circuit accommodation portion 3 of a detector body 2 where the summit thereof is brought to the outside. The heat sensor element 1 and the portion of a hole on the detector body 2 through which the element is brought to the outside are formed into a sealed portion 5, for example, by potting of adhesives or the like. It should be noted that, instead of potting of adhesives, a packing may be used.
A back side cover 6 is mounted on the reverse side of the circuit accommodation portion 3 via a rubber packing 7 or the like. Provided on the reverse side of the back side cover 6, are a pair of fitting members 8 which are electrically and mechanically connected to a detector base.
Mounting of the printed circuit board 4 and the fitting members 8 onto the back side cover 6 is effected by using screws 9 and cylindrical contacting members 10. That is, they are fixed to the back side cover 6 by tightening the screw 9 in the state where the contacting member 8 is placed between the printed circuit board 4 and the fitting member 10.
However, in the construction of such conventional sensor, there are problems, as follows.
Firstly, a conventional sensor is constructed to have an airtight structure by using potting or packing in the state where the heat sensor element 1 connected to the circuit board 4 by thin lead wire, is brought out to the outside thereof so as to keep the airtightness of the circuit accommodation portion 3. For this reason, manual work must be performed, since the lead wire of the heat sensor element 1 is thin and is difficult to be handled. Thus automatization of assembling process thereof is difficult. Further, in the case of potting where adhesives or the like is used, time is also required until it is dried and fixed.
Secondly, mounting of the circuit board 4 and the fitting member 8 with respect to the back side cover 6 is also performed by a manual operation, because the screw 10 is used. Further, a gap occurs at the mounting portion. Airtightness within the detector cannot thus be adequately secured.
Thirdly, in assembling of the body, printed circuit board and cover of a conventional thermal detector, they are fixed by screw 6 in the state where threaded-holes and threaded through holes formed respectively on them are positioned with respect to each other. For this reason, the screwing operation must be performed at the same time of their positioning. Assembling operation of the detector is thus complicated and, in addition, variance in positioning due to the condition of screwing is large. Especially when automatization of assembling work by a robot or the like is attempted, the yield of products is also reduced, since the working process thereof is complicated and requires high positioning accuracy.